Multipactor-Triggering Powers’ Modelling of a WEST ICRH Antenna During RF Conditioning

Abstract

During radio frequency (RF) conditioning of the ion cyclotron resonance heating (ICRH) system on WEST, under vacuum, pressure rises are measured inside the antennas. A proposed hypothesis for the cause of this phenomenon is multipactor, an electron multiplication process taking place in RF devices under vacuum. Modelling multipactor conditions in a resonant antenna, i.e., under high standing wave (SW) ratio, requires particular precautions. This article proposes a methodology to determine the multipactor conditions in such a context. Using the proposed approach, the operational conditions, expressed as the generator’s RF powers, of the appearance of multipactor inside a WEST ICRH antenna during its RF conditioning under vacuum, are deduced. Calculations are performed for two different surface states: when the surface is baked but nonconditioned, and when it is baked and fully conditioned. For the nonconditioned surface state, it is shown that when only one side of the under-conditioning ICRH antenna is powered, the off-mode side is prone to multipactor when its capacitors are tuned. Detuning the capacitors on the off-mode side suppresses almost the multipactor on the nonpowered side for the frequency range of interest. Moreover, it is shown that the fully conditioned antenna surface state can suppress the appearance of multipactor in some regions of the antenna.